The present invention relates to an apparatus and method for accurately delimited cryoablation of unwanted body tissues. More particularly, the present invention relates to method and apparatus for cryoablating a selected target volume of body tissue while surrounding or partially surrounding said target volume with a protective envelope of mildly heated tissue, so as to enhance accuracy of delimitation of the volume of cryogenic destruction, and so as to minimize the volume of tissue, exterior to the selected target volume, damaged by cryogenic cooling.
In recent years, cryoablation of tissues has become an increasingly popular method of treatment for a variety of pathological conditions. Malignancies in body organs such as the breast, prostate, kidney, liver, and other organs are successfully treated by cryoablation, and a variety of non-malignant pathological conditions, such as benign prostate hyperplasia, benign breast tumors, and similar growths are also well treated by cryoablation of unwanted tissues. Certain cases of intractable chronic pain are also treatable through cryosurgery, by cryoablation of selected nervous tissue.
Cryoablation of pathological tissues or other unwanted tissues is typically accomplished by utilizing imaging modalities, such as x-ray, ultrasound, CT, and MRI, to identify a locus for ablative treatment, then inserting one or more cryoprobes into that selected treatment locus, then cooling the treatment heads of the inserted cryoprobes sufficiently to cause the tissues surrounding the treatment heads to reach cryoablation temperatures, typically below about −40° C.
Tissues thus cooled are thereby caused to loose their functional and structural integrity. Cancerous cells cease growing and multiplying, and cryoablated tumor tissue materials, whether from malignant tumors or from benign growths, lose their structural integrity and are subsequently sloughed off or absorbed by the body.
The principle danger and disadvantage of cryosurgical ablative treatment, however, is the danger of partially or completely destroying the functional and structural integrity of healthy tissues near the treatment locus, thereby impeding the patient's recovery from the surgical procedure and potentially causing serious and long-term deleterious effects on the patient's health and on his quality of life.
In particular, two well-known limitations inherent in currently known cryoablation technique are primarily responsible for damage caused to healthy tissue while cryoablating pathological tissue.
Using terms defined hereinbelow, we would say that the first problem is that in all cryoablation the “ablation volume”, a first volume within which tissue structure and functionality are destroyed, is inevitably surrounded by a “damage envelope”, a second volume within which tissue structure and function are damaged. Tissues in the damage envelope are exposed to temperatures which, although not sufficiently cold to thoroughly cryoablate those tissues and wholly destroy their physiological functionality, yet are cold enough to do significant damage to those tissues, impair their functionality, and significantly alter cellular and other structures therein. To reliably ablate a first selected target volume of tissue, one is inevitably obliged to damage second volume of tissue surrounding that first selected volume.
The second problem is that cryosurgery is difficult to control, because the border between the ablation volume and the damage envelope is not directly visible under any known imaging modalities. Although the borders of the ice-ball which forms around the cold operating tip of a functioning cryoprobe is visible under ultrasound or MRI imaging modalities, the border of the ablation volume, the volume within which cell functionality is reliably destroyed, is itself not directly visible under known imaging modalities, and it's position, somewhere within the visible ice-ball, must be estimated or indirectly detected or guessed.
Various devices and methods have been proposed to enable cryoablation of pathological tissue while limiting damage to non-pathological tissue. These fall roughly into two categories: devices and methods which protect tissues by preventing excessive cooling of those tissues during a cryoablation procedure in their vicinity, and devices and methods which enable accurate placement of cryoprobes used in cryoablation, so as to successfully concentrate the cooling effect of such cryoprobes at or near pathological tissue, thereby minimizing unwanted cooling of non-pathological tissue.
An example of the former category is the well-known technique of introducing a heating device or a heated fluid into the urethra of a patient, thereby heating the urethra and tissues adjacent to it during cryoablation of portions of the prostate, thereby helping to protect the urethra from damage while prostate tissues nearby are being cooled to cryoablation temperatures. U.S. Pat. No. 6,505,629 to Mikus et. al. teaches a similar method, using a heating probe to protect an object, the neuro-vascular bundle, during cryoablation of the prostate, by interposing a heating probe between that object and a cooling cryoprobe.
An example of the latter category is provided by U.S. Pat. No. 6,142,991 to Schatzberger. Schatzberger describes a high resolution cryosurgical method and device for treating a patient's prostate, including the steps of (a) introducing a plurality of cryosurgical probes to the prostate, the probes having a substantially small diameter, the probes being distributed across the prostate, so as to form an outer arrangement of probes adjacent the periphery of the prostate and an inner arrangement of probes adjacent the prostatic urethra; and (b) producing an ice-ball at the end of each of the cryosurgical probes, so as to locally freeze a tissue segment of the prostate. Schatzberger's apparatus includes (a) a plurality of cryosurgical probes of small diameter, the probes being for insertion into the patient's organ, the probes being for producing ice-balls for locally freezing selected portions of the organ; (b) a guiding element including a net of apertures for inserting the cryosurgical probes therethrough; and (c) an imaging device for providing a set of images, the images being for providing information on specific planes located at specific depths within the organ, each of the images including a net of marks being correlated to the net of apertures of the guiding element, wherein the marks represent the locations of ice-balls which may be formed by the cryosurgical probes when introduced through the apertures of the guiding element to the distinct depths within the organ.
Thus, Schatzberger's method and apparatus enable a surgeon to place a set of cryoablation probes within a prostate with relatively high accuracy, and to operate those probes to ablate selected tissues while avoiding, to a large extent, inadvertent and undesirable ablation of healthy tissues near the ablation site. Schatzberger also demonstrates that by utilizing multiple small cryoprobes in a dense array, the volume of the damage envelope may to some extent be reduced.
However, neither Schatzberger's technique nor any other known technique has proven sufficiently accurate to prevent damage to peripheral tissues in general. An ablation target ablated according to the methods of Schatzberger is still surrounded by broad envelope of damaged tissue. Further, Mikus' invention, while solving the specific problem of unwanted damage to a specific object, does not address the general problem of the overall “sloppiness” of the cryoablation procedure. Cryoablation, as practiced under all known prior art methods, results in cryoablation of a first volume, only approximately conforming to an intended cryoablation target, which first volume is surrounded by a second volume of healthy tissue, unavoidably damaged.
Thus there is a widely recognized need for, and it would be highly advantageous to have, apparatus and method for cryoablation which results in reduced volume of damaged tissue surrounding the selected cryoablation target, yet enables full and reliable cryoablation of the selected target.
As mentioned above, a second basic problem in cryosurgery technology relates to the difficulty experienced by surgeons in knowing the exact extent of the tissue which will be ablated by a given cryoablation procedure. The ice-ball produced by a functioning cryoprobe is visible under ultrasound and other imaging modalities, but the delimitation of the cryoablation volume (the area of total cell destruction) within that iceball is not directly visible under known imaging technologies. The surgeon, who in the case of treatment of a malignancy must err on the side of caution, often ablates more tissue than was really necessary, and damages more additional tissue than was really necessary, because he is unable to accurately command the exact delimitation of the destruction volume he creates, and is further unable to accurately observe, in real time, the actual border of the destruction volume created by his cryoablative intervention.
Thus there is a widely recognized need for, and it would be highly advantageous to have, a cryosurgery apparatus and method enabling accurate delimitation of an ablation volume.
With respect to prior art relevant to another aspect of the invention, Mikus op. cit. teaches use of low-pressure helium supplied to a cryoprobe having a Joule-Thomson orifice, to supply heating to a probe. According to Mikus, low-pressure helium is used in place of high-pressure helium, to assure that a tissues will not be heated beyond a temperature which would be destructive to those heated tissues.
Use of low-pressure helium for heating a Joule-Thomson probe does indeed ensure that a desired maximum temperature of the probe will not be exceeded. There is, however, a disadvantage to use of low-pressure helium for heating such a probe, namely that the heating capacity of a probe so heated is somewhat limited. Use of low pressure of the supplied helium, supplied through a small-diameter gas-supply conduit, insures that only a relatively small quantity of helium gas will be passed through the Joule-Thomson orifice per unit of time. This limitation is particularly noticeable when the method is applied to probes of small dimensions. Yet, as taught by Schatzberger op. cit., small-diameter cryosurgical devices are desirable in many cryosurgery contexts, and small diameter cryoprobes comprise even smaller diameter gas input supply conduits. Thus, use of low-pressure helium to heat today's miniaturized cryoprobes substantially limits the heating ability of such a probe.
Thus, there is a widely recognized need for, and it would be highly desirable to have, a device and method for Joule-Thomson heating of a probe, which device and method provide heating to an upper limit of temperature, thereby protecting heated tissues from overheating, provide for a high throughput of gas, and therefore provide a higher heating capacity than that provided by a Joule-Thomson probe heated by expansion of low-pressure helium gas.
Note is here taken of three additional prior art documents presenting devices or methods having elements in common with devices and methods presented herein, or presenting devices for which new uses are presented hereinbelow.
First, Zvuloni et. al. in U.S. patent application Ser. No. 10/255,834 (Publication No. 2003-0060762-A1) teaches use, in a cooling cryoprobe, of a gas mixture comprising both a cryogenic cooling gas and a heating gas such as helium. Zvuloni contemplates use of such a gas so as to enable fine control of cooling, and to enable leak detection in a balloon catheter based on detection of trace amounts of helium.
Second, in PCT application IL02/01062 Zvuloni et. al. teach use of a cryoprobe having a cooling tip and a heated shaft, operable to protect tissues adjacent to the shaft of such a probe, which shaft, absent a heating or insulating effect in the shaft, would in some circumstances be sufficiently cooled by passage therein of exhaust cooling gasses from the probes's cooling tip to risk damaging, by cooling, healthy tissues adjacent to that shaft.
Third, in U.S. Pat. No. 6,074,412, Mikus et. al. teach a probe having both heating elements and cooling elements, yet the heating and cooling elements of Mikus' probe are designed for, and can only be used, sequentially and not simultaneously.